Biochemically, the cerebellum is characterized by a large concentration of cGMP, cAMP, and cGMP-dependent protein kinase. Drug-induced changes in GABA receptor function can be easily monitored by measuring changes in these biochemical paremeters. It is suggested that diazepam and muscimol (a modulator and a direct GABA receptor agonist, respectively), modulate cerebellar function by altering the cGMP system. We are now investigating whether diphenylhydantoin has its own receptor on cerebellar structures and which are the short and long term biochemical changes associated with the stimulation of this receptor.